run-tests: setup "correct" python earlier

I don't see any good reason to wait. This result in  slightly simpler code.

Differential Revision: https://phab.mercurial-scm.org/D11047

tests/run-tests.py

@@ -3419,11 +3419,10 b' class TestRunner(object):'
             if self.options.list_tests:
                 result = runner.listtests(suite)
             else:
+                self._usecorrectpython()
                 if self._installdir:
                     self._installhg()
                     self._checkhglib("Testing")
-                else:
-                    self._usecorrectpython()
                 if self.options.chg:
                     assert self._installdir
                     self._installchg()
@@ -3705,8 +3704,6 b' class TestRunner(object):'
             sys.exit(1)
         os.chdir(self._testdir)
 
-        self._usecorrectpython()
-
         hgbat = os.path.join(self._bindir, b'hg.bat')
         if os.path.isfile(hgbat):
             # hg.bat expects to be put in bin/scripts while run-tests.py